The present invention is directed to a process for recording low-frequency, wide-band signals on thermoplastic storage medium and particularly for recording electrical interference signals from acoustic holograms on thermoplastic material.
The use of thermoplastic material as a storage medium for recording holograms is known. For example, the process for recording holograms on a thermoplastic storage medium is discussed in a German Language Book, Kiemle and Roess Einfuehrungin Die Technik Der Holografie, Akademische Verlagsgesellschaft Frankfurt-Main, 1969, page 217. To use the thermoplastic material, it is usually applied in a thin layer on a transparent photo conducting carrier, such as photo conductive glass. The outer surface of the thermoplastic material is sensitized by providing it with a uniformly distributed positive or negative charge by means of a corona charge.
During exposure, the photo conductive carrier becomes conductive in those areas or points on which light falls but at the unexposed points it remains in an insulative or nonconductive condition. Thus, the exposing creates a charge profile corresponding to the light interference pattern projected onto the storage medium.
To develop the thermoplastic material, it is heated to a temperature which is either a softening point or partial melting point. As it reaches its melting point, those particular areas which have charges on opposite surfaces which charges have opposite polarity are drawn together by the attraction of the charges to cause the formation of a surface relief corresponding to the charge distribution while those areas without the charge pattern remain substantially unaffected. Upon cooling, the surface relief formed by the distribution of charge particles is fixed and in this way a surface-phase hologram can be recorded on the thermoplastic storage material.
The information or hologram can be erased from the storage material if the material is heated to above the softening point. At this temperature, the surface tension of the material forms a smooth surface layer. After the erasing step, the synthetic material can be used again for recording other data.
In comparison to other holographic recording materials, thermoplastic materials have some disadvantages. For example, they ony record a limited spatial frequence band around a center frequency. The band width, which is mainly dependent upon the thickness and surface tension of the thermoplastic material and on the charge distribution, influences the holographic recording depending upon the spatial frequency.
Sonar signals are produced in acoustic holography. For example, the amplitude and phase distribution of a sound field, which is produced by acoustically, coherently irradiated object, is scanned in a receiving plane by a transducer which converts the sound field to electrical signals. A coherent reference signal is added to the received signal and the electrical interference signal formed in this way is conducted to electro-optical transducer, for example an oscilloscope, which converts the electrical signals into a visual presentation. The sound receiver can be either a matrix of sound transducers which are electronically scanned or an individual sound transducer which is moved mechanically over the receiving plane and thus scans the sound field row-by-row in predetermined intervals.
If acoustical holograms are to be portrayed on thermoplastic material for the purpose of an optical short-time reconstruction, a problem occurs in that the data which is to be recorded is generally of very low frequency. Whereas approximately 500 lines/mm can usually be recorded by holographic recording on a thermoplastic material, one line/mm is an average for the recording of acoustical holograms on thermoplastic storage material. Since the resonance frequency of thermoplastic storage material is at considerably higher frequencies, a very poor signal-to-noise ratio will be obtained.